Stabilized starch, use of the stabilized starch and methods of producing it

ABSTRACT

The method of making a stabilized starch includes mixing an aqueous starch suspension having a starch concentration of from 5 to 50 % by weight with an aqueous metal salt solution which contains from 10 to 50 % by weight of at least one metal salt selected from the group consisting of TiOSO 4 , TiOCl 2 , Al 2  (SO 4 ) 3 , NaAlO 2 , Fe(NO 3 ) 3 , FeSO 4 , ZnCl 2 , Na 2SiO   3 , SbCl 3 , ZrOSO 4 , ZrOCl 2 , MgCl 2  and SnCl 4  with a starch-suspension-to-metal-salt-solution ratio of from 1:5 to 10:1, to form a mixture having a pH of from 3 to 10 and containing the stabilized starch; and after the mixing of step a), separating the stabilized starch from the mixture and washing and drying it. The starch suspension preferably contains starch particles having a particle size of from 1 to 150 microns.

The invention is directed to a stabilized starch, use of the stabilizedstarch and methods for producing it.

As a natural and inexpensive raw material, starch has many industrialuses. In addition to the use of starch in the foodstuffs industry, e.g.,for fabricating starch sugar, glucose syrup, dextrin, puddings, potatosago and licorice, starch is used in the production of pastes and glues,as paper additives, e.g. as sizes for paper, as thickening agents forprinting inks and as soap additives. Further, starch products, asnatural polymers, are combined with synthetic polymers to improve theircharacteristics. However, in so doing, the mechanical properties of thesynthetic polymers are disadvantageously impaired by combining withstarch. Moreover, starch decomposes in its monosaccharides at hightemperatures so that it is only possible to use copolymers formed fromsynthetic polymers and starch at temperatures below approximately 190°C. In addition, starch has a relatively low resistance to chemicals,which causes problems in the industrial use of starch.

The object of the invention is to provide a stabilized starch which hasa high thermal, mechanical and chemical stability and can be used inmany ways. Further, the invention also has the object of providingmethods for the production of stabilized starch which can be carried outeasily and quickly.

The object upon which the invention is based is met in that thestabilized starch is formed from a substrate of starch which issurrounded by a protective layer of inorganic metal compounds.

By starch is meant amylose or amylopectin or a mixture of amylose andamylopectin. Amylose is unbranched and contains on the average 300glucopyranose molecules. Amylopectin is composed of branchedmacromolecules with over a thousand glucose molecules. Rice starch,potato starch, wheat starch, and preferably maize or corn starch can beused as a mixture of amylose and amylopectin. Rice starch, potatostarch, wheat starch and corn starch are generally composed of 20 to 30%amylose and 70 to 80% amylopectin. Metal oxides, metal hydroxides, metalphosphates, metal sulfides or metal carbonates, for example, can be usedas inorganic metal compounds. Surprisingly, it has now been shown thatthe stabilized starch achieves high thermal, mechanical and chemicalresistance as a result of the protective layer without modification ofthe polymer structure of the starch. The porosity, coarseness,absorption capacity and dispersion capacity of the stabilized starch canbe adapted to a relatively great number of applications in anadvantageous manner through the selection of the material of theprotective layer and selection of the thickness of the protective layer.

In a preferred form of the invention, TiO₂ or TiO(OH)₂ or Al₂ O₃ or Al₂(OH)₆ or FeO or Fe₂ O₃ or Fe(OH)₃ or ZnO or ZnO(OH)₂ or ZnS or ZrO₂ orZr(OH)₄ or ZrO(OH)₂ or MgO or Mg(OH₂ or SnO₂ or Sn(OH)₄ or SiO₂ or BaSO₄or Sb₂ O₃ or mixtures thereof are used as inorganic metal compounds. Ahigh dispersion capacity of the stabilized starch in plastics isachieved by this step.

Another preferred form of the invention consists in that the proportionof the protective layer on the stabilized starch is 0.5 to 40 percent byweight. A particularly high mechanical stability of the starch isachieved by this step.

According to another form of the invention, the proportion of theprotective layer on the stabilized starch is 1 to 30 percent by weight.The stabilized starch accordingly has a relatively high thermalstability and the proportion of the protective layer on the stabilizedstarch is relatively small so that the price of the stabilized starchcan be kept low without negatively affecting the quality of thestabilized starch.

The object upon which the invention is based is met also in a method forproducing stabilized starch in which an aqueous starch suspension with astarch concentration of 5 to 50% is mixed with an aqueous metallic saltsolution containing TiOSO₄ or TiOCI₂ or Al₂ (SO₄)₃ or NaAlO₂ or Fe(NO₃)₃or FeSO₄ or ZnCl₂ or Na₂ SiO₃ or SbCl₃ or ZrOSO₄ or ZrOCI₂ or MgCI₂ orSnCl₄ or mixtures thereof in a metallic salt concentration of 10 to 50percent by weight, at a pH of 3 to 10 in a ratio of the starchsuspension to the metallic salt solution of 1:5 to 10:1 and thestabilized starch is then separated, washed and dried. Surprisingly, ithas been shown that the individual particles of the starch suspensioncan be enveloped uniformly with a closed protective layer by using thismethod. The stabilized starch produced by this method therefore has aparticularly high chemical stability.

The object upon which the invention is based is further met by a methodfor producing stabilized starch in which an aqueous starch suspensionwith a starch concentration of 5 to 50% is mixed with a secondsuspension containing particles of TiO₂ or TiO(OH)₂ or TiO(OH)₄ or ZnOor ZnS or BaS₄ or Fe(OH₃ or Fe₂ O₃ or SiO₂ or Al₂ O₃ or AI(OH)₃ or Sb₂O₃ or ZrO₂ or mixtures thereof in a concentration of 10 to 50 percent byweight, at a pH of 4 to 9 in a volume ratio of the starch suspension tothe second suspension of 1:1 to 10:1 and the stabilized starch is thenseparated and dried. The advantage of this method consists in that astabilized starch can be produced in which the individual particles inthe protective layer are arranged relatively densely by addition to thesubstrate of starch so that tile stabilized starch also has aparticularly high mechanical stability in addition to a relatively highthermal and chemical stability.

According to another form of the invention, the pH is 5 to 8. This hasthe advantage that the starch substrate can be surrounded by aprotective layer in a relatively short amount of time.

According to another form of the invention, the particle size of thestarch suspension is 1 to 150 μm. If the size of the particles of thestarch suspension is 1 to 150 μm, a stabilized starch can be producedwhich can be combined with synthetic polymers in a simple manner.

Another form of the invention consists in that the size of the particlesof the starch suspension is 5 to 25 μm. In this way, a stabilized starchcan be produced for a relatively great number of uses.

According to another form of the invention, the average particle size inthe second suspension is 0.02 to 3 μm. In this way, addition of theparticles of the second suspension to the starch substrate can beeffected in a uniform manner so as to achieve a homogeneous distributionof the particles of the second suspension in the protective layer.

According to another form of the invention, the average particle size ofthe second suspension is 0.02 to 1 μm. This has the advantage that theparticles of the second suspension are arranged homogeneously in theprotective layer and the protective layer has a relatively high density.

According to another form of the invention, the stabilized starch isused as filler or pigment in plastics, paper, shellacs, paints,cosmetics, photochemical materials or pharmaceutical products. Thequality of plastics, paper, shellacs, paints, cosmetics, photochemicalmaterials or pharmaceutical products can be improved in this way.

The invention is explained in more detail with reference to thefollowing examples.

EXAMPLE 1

100 g corn starch with a particle diameter of 10 to 25 μm are stirredinto 400 ml water. The starch suspension obtained in this way has a pHof 5.1. 160 ml of a suspension containing 15 percent by weight TiO₂ andhaving a pH of 6.3 is then added to the starch suspension during 5minutes. The mixture is stirred for 1 hour. The stabilized starch isthen filtered and dried.

EXAMPLE 2

To produce a titanium oxide hydrate suspension, 1000 ml of ahydrochloric acid solution containing 85 g of TiOCl₂ and having a pH of1 are neutralized to pH 7 with 1500 ml of 10-percent NaOH over a periodof 50 minutes, then filtered and washed. The salt-free titanium oxidehydrate and water produce a titanium oxide hydrate suspension with aconcentration of 10 percent by weight with respect to the TiO₂ content.100 ml of this suspension are stirred into a starch suspensioncontaining 100 g corn starch in 400 ml water over a period of 5 minutes.The mixture is stirred for 60 minutes. The stabilized starch is thenseparated and dried at 105° C.

EXAMPLE 3

100 g of corn starch are dispersed in 500 ml water. The pH of thesuspension is adjusted to 7 with 1 ml of a 10-percent NaOH solution.Following this, 110 ml of a TiOCl₂ solution containing 18.6 g TiOCl₂ isadded during 45 minutes. A 10-percent NaOH solution is added at the sametime so that the pH is adjusted to 6 to 8. Alter stirring for 60minutes, the stabilized starch is filtered, washed and dried.

EXAMPLE 4

250 g of corn starch are dispersed in 500 ml water. 130 ml of asuspension containing 37.5 g TiO₂ with an average particle size of 0.1μm are added to this starch suspension. After stirring for 1 hour, theproduct is filtered and dried.

EXAMPLE 5

A suspension of 250 g of corn starch in 500 ml water is produced. Then,150 ml of a suspension containing 40 g of ZnO are added over a period of5 minutes. The stabilized starch is filtered and dried after stirringfor 60 minutes.

EXAMPLE 6

250 g of corn starch are dispersed in 500 ml water. A ZnS suspensioncontaining 44.4 g of ZnS in 120 ml water is then added to this starchsuspension during 5 minutes. After stirring for 60 minutes, thestabilized starch is filtered and dried.

EXAMPLE 7

400 g of corn starch are dispersed in 8000 ml water. 7200 ml of atitanium oxide hydrate suspension with a concentration--with respect tothe TiO₂ content--of 7.7 percent by weight are then added over a periodof 45 minutes. The stabilized starch is then spray dried. The driedstabilized starch contains 12 percent by weight TiO₂ and has a specificsurface of 41 m² /g which is determined by the BET method. Examinationof the stabilized starch under a scanning electron microscope revealsthat the corn starch is surrounded by a protective layer of TiO₂. Foranalysis of mechanical stability, 20 g of stabilized starch are milledin a ball mill for 2 hours using porcelain balls with a diameter of 20mm. Examination of the milled stabilized starch by a scanning electronmicroscope reveals that the corn starch is still almost completelyenveloped by the protective layer of TiO₂ even after the mechanicalloading in the ball mill. In contrast to untreated corn starch, thedried stabilized starch can easily be worked into polyethylene, sincethe protective layer of TiO₂ prevents the decomposition of the cornstarch as a result of the high temperatures of more than 190° C. atwhich it is worked into polyethylene. Untreated corn starch decomposeswhen worked into polyethylene, which results in an undesirable browncoloring of the polyethylene mixed with starch.

What is claimed is:
 1. A method of making a stabilized starch, saidmethod comprising the steps of:a) mixing an aqueous containing starchparticles having a starch concentration of from 5 to 50% by weight withan aqueous metal salt solution, said aqueous metal salt solutioncontaining from 10 to 50% by weight of at least one metal salt selectedfrom the group consisting of TiOSO₄, TiOCl₂, Al₂ (SO₄)₃, NaA1O₂,Fe(NO₃)₃, FeSO₄, ZnCl₂, Na₂ SiO₃, SbCl₃, ZrOSO₄, ZrOCl₂, MgCl₂ andSnCl₄, wherein said aqueous starch suspension and said aqueous metalsalt solution are mixed in a ratio of said aqueous starch suspension tosaid metal salt solution of from 1:5 to 10:1, wherein said at least onemetal salt forms a protective layer on said starch particles in saidstarch suspension to form a stabilized starch mixture, said starchmixture having a pH of from 3 to 10 and containing a stabilized starch;b) after said mixing of step a), separating said stabilized starch fromsaid mixture and washing and drying said stabilized starch separatedfrom said mixture.
 2. A method as defined in claim 1, wherein said pH ofsaid mixture is between 5 and
 8. 3. A method as defined in claim 1,wherein said starch particles have a particle size of from 1 to 150microns.
 4. A method as defined in claim 1, wherein said starchparticles have a particle size of from 5 to 25 microns.